Colorimeters have employed white incandescent light and utilized color filters to select the proper color range for absorption. The choice of incandescent light is fraught with error because incandescent light is power consuming, varies greatly with small changes in voltage, and becomes less bright with time due to deposit of tungsten on the transparent quartz or glass transmitter.
Absorption spectroscopy both in the visible and infrared range has proved extremely valuable for the identification of compounds in solution. The limitations and usefulness of absorption spectroscopy has been covered in many texts eg.: Melon, Analytical Absorption Spectroscopy, John Wiley, NY 1950; Edisbury, Practical Hints on Absorption Spectroscopy, Plenum Press, 1968. The widespread use of absorption spectroscopy has lead to the publications of absorption spectra for almost all known compounds, eg.: Lang, L. Absorption Spectra in the Ultraviolet and Visible Region, Academic Press NY; Data for organic compounds can also be found in "Organic Electronic Spectral Data" by Phillips, Lyle & Jones Interscience Publishers NY. It thus becomes a simple matter to find the absorption spectra of any known substance or indicator. The indicators include those for oxidation-reduction (redox), pH and specific chemicals. When the absorption spectra is known, different frequencies on the absorption curve can be chosen and the proper L.E.D. selected so that the emission spectrum narrowly covers some selected frequency of the absorption spectra.
The absorption for Malachite, a commercial green dye, is maximum at 6000 .ANG. in the orange-red range therefore the solution appears green. This color is the complimentary colors which is not absorbed.
A lesser secondary peak at 4000-4500 .ANG. appears in the violet zone. Very dilute solutions would best be determined with a red or orange emitting L.E.D. to match the absorption peak at 6000 .ANG.. Yet, high concentrations of malachite would be almost totally absorbed at this frequency. Therefore, it would be better to select a violet emitting diode if one wanted to determine high concentrations with less accuracy. If a red L.E.D. were used, dilute concentrations might be more exactly determined but at elevated concentrations the lesser absorption peak would be desirable and the emission spectra automatically switched.
In 1852, Beer observed that for any given thickness of a solution the transmittance of light of any specific wavelength depended exponentially on the concentration of the absorbing species. Thus if one were to plot transmittance against concentration for any specific substance at a specific wavelength, a linear relationship would exist between concentrations and transmittance and the plot would be a straight line. The slope of this straight line would be determined by the specific absorbance of the substance at the particular wavelength of the emitter. When the specific absorbance at one frequency is high, the slope, of the line is steep and with lower absorbance at a different frequency it would be more gradual. Obviously, if one wishes to accurately determine the substance at low concentrations the frequency with the steep slope is most desirable. If one wished to determine differences at high concentrations, a lower absorption peak would be preferable.
This problem of sensitivity for high and low ranges has previously been solved by having two different ranges on the same paper strip each with different color changes. This involves two independent circuits with two concentrations placed on the strip. Automatic switching to another L.E.D. better solves this problem without having to set two separate ranges on a single paper strip.